Is it ok to hookup with my girlfriends mom

Recent as I'm visiting my girlfriend, her mom is trying to get physical with me. Leaving her hand on my thigh, coming to sit close to me while I'm sited on couch, she even walked up to me naked one morning when I stayed there which was very intentional according to the circumstances but I freaked out at that moment. Such instances have happened multiple times. Haven't informed my girlfriend about all of it. But her mom has perfect body and face and tbh I had fantasized about her. What should I do?